1) Field of the Invention
The present invention relates to a process for surface treating a titanium-containing metallic material. More particularly, the present invention relates to a process for surface treating a titanium containing metallic material to form a composite coating layer having an excellent heat resistance, abrasion resistance, and optionally, a high sliding property, and closely adhered to a surface of the titanium-containing metallic material surface.
2) Description of the Related Arts
It is known that various titanium-containing metallic materials, for example, titanium or titanium alloy materials, are usable for producing various valve parts and driving system parts of automobiles and autobicycles, for example, engine valves, valve springs, valve retainers, connecting rods, rocker arms and valve lifters, which must be light, and parts of pumps for chemical industries, which must have a high resistance to corrosion.
The titanium-containing metallic materials frequently must have a high heat resistance and abrasion resistance, and optionally, an excellent sliding property.
In the conventional titanium-containing metallic materials, the abrasion resistant coating layer is formed by dry plating methods, for example, gas nitriding method, salt bath nitriding method, ion-nitriding method, ionplating method, chemical vapor deposition (CVD) method and physical vapor deposition (PVD) method, or by wet plating methods including a pre-treating step by a Marchall method, Thoma method or ASTM method.
The above-mentioned conventional nitriding methods are disadvantageous in that the treated material is greatly deformed due to a high treating temperature, which causes a high thermal strain of the material, and that it takes a long time to form the nitrided hard layer, and thus the productivity of the hardened layer is low.
Also, the conventional dry and wet plating methods are disadvantageous in that the resultant coating layer exhibits a low adhering strength to the titanium or titanium alloy material, and thus is easily separated during practical use.
This easily separable coating layer cannot exhibit a high resistance to severe wear conditions.
Namely, a high wear resistant coating layer should have a high abrasion resistance, a high sliding property, and a high close adhering property to the titanium-containing metallic material surface.
Japanese Unexamined Patent Publication No. 1-79,397 discloses a process for forming a high abrasion-resistant coating layer on a titanium or titanium alloy material by utilizing a Martin-Thoma method.
This process is disadvantageous in that, since a heat-treatment in an oxidative gas atmosphere is applied to a titanium or titanium alloy material plated with a metal, for example, nickel, by a chemical deposition method, the plated metal layer is oxidized in the heat treatment, and thus the oxidized portion of the plated metal layer must be eliminated before an additional metal coating layer, for example, a chromium coating layer, is formed on the metal (nickel) coating layer. Also, this additional chromium coating layer, which forms an outer most layer of the surface treated material exhibits a poor anti-seizing property and unsatisfactory heat and abrasion resistances.